Most vascular prostheses in current clinical use are formed of knitted or woven fabric with open interstices. Those interstices must be closed by thrombus, either before or after implantation. The tubular wall of such a prosthesis thus becomes sealed by a fabric-thrombus complex which is permeable to cellular ingrowth and which serves as the ground substance for subsequent endothelialization or healing of the graft. L. R. Sauvage et al., Surg. Clin. N. Am., 54:213-228 (1974).
In general, such grafts, whether woven or knitted, and whether formed of any of a variety of polymeric materials, have been used effectively only in connection with relatively large arteries (i.e., arteries having outside diameters of approximately 10 to 30 millimeters) such as, for example, the abdominal aorta and the aorto-iliac or aorta-femoral branches. Efforts to use such grafts, either as replacement or bypass grafts, with medium-sized arteries have been associated with a high incidence of delayed thrombotic occlusion, primarily because the whole blood thrombus which effectively seals the interstices of the graft is itself highly thrombogenic and causes additional thrombus which ultimately destroys the patency of the prosthesis. For the same reason, such grafts are regarded as totally unacceptable for small vessels (i.e., vessels having inside diameters less than about 8 millimeters).
While it has been suggested that problems of thrombotic occlusion might be avoided by forming the vascular prosthesis from a blood-tight biocompatible material, no such material meeting all of the requirements for an impervious graft (e.g., non-thrombogenic, highly conformable, readily suturable, an outer surface susceptible to tissue ingrowth, an inner surface to which any thrombus that does form can securely attach to discourage embolization, sufficient strength for easily withstanding hemodynamic pressures), has been reported. Some materials, such as expanded polytetrafluoroethylene, have met many of such requirements but, because of thrombogenicity or some other failing, have been found unacceptable. In the absence of an effective non-thrombogenic blood-impermeable material from which to form such a graft, the accepted approach has been to form vascular grafts from open mesh thrombogenic materials (polytetrafluoroethylene, polyethylene terephthalate, nylon, etc.) which depend for their effectiveness on thrombus formation (to render the grafts blood-tight) followed by endothelialization of the thrombus (to render the interior surfaces thrombus resistant). For reasons already given, the net result is that synthetic vascular prostheses are not currently available, or at least not recommended, as grafts for medium and small caliber vessels.
Even for larger caliber vascular grafts, where a porous thrombogenic material such as knitted polyethylene terephthalate (Dacron) has been commonly used, studies have revealed that only in relatively few cases do clinical grafts attain a completely healed flow surface, that is, a surface where cellular ingrowth has completely covered the luminal surface of the grafts to produce a continuous non-thrombogenic lining. While it has been suggested that some synthetic material of low thrombogenicity such as a hydrogel might be coated upon the luminal surfaces to insure a continuous thrombus-resistant lining, the absence of reports of successful experiments supports earlier appraisals that hydrogels, despite their desirable properties, do not have the mechanical strength and perhaps other requirements for effective use in vascular prostheses.
Additional references illustrative of the state of the art are: Greer, R. T., B. H. Vale, and R. L. Knoll, "Hydrogel Coatings and Impregnations in Silastic, Dacron, and Polyethylene," in Scanning Electron Microscopy, Vol. I, 1978, ed. by O. Johari, SEM Inc., Chicago, pp. 633-642; Ratner, B. D., A. S. Hoffman, "Radiation Grafted Hydrogels on Silicone Rubber as New Biomaterials," in Biomedical Applications of Polymers, ed. by H. P. Gregor, Plenum Publishing Corporation, New York, N. Y., pp. 159-171, 1975; Predecki, P., "A Method for Hydron Impregnation of Silicone Rubber, "J. Biomedical Materials Research, J. Biomed. Mater. Res. Vol. 8, pp. 487-489, 1974; Campbell, C. D., D. Goldfarb, and R. Roe, "A Small Arterial Substitute," Ann. of Surg., Vol. 182, pp. 138-143, 1975; Ratner, B. D., and A. S. Hoffman, "Synthetic Hydrogels for Biomedical Applications," in Hydrogels for Medical and Related Applications, American Chemical Society Serial No. 31, ed. by J. D. Andrade, A.C.S., Washington, D. C., pp. 1-36, 1976; Lindenauer, S. M., T. R. Weber, T. A. Miller, S. R. Ramsburgh, C. A. Salles, S. P. Kahn, R. S. Wojtalik, "The Use of Velour as a Vascular Prosthesis," Biomed. Eng., pp. 301-306, Sept. 1976; U.S. Pat. No. 2,976,576.